Magnetoresistive sensors such as tunnel magnetoresistance (TMR) sensors and giant magnetoresistance (GMR) sensors are frequently employed as sensing elements in the magnetic read heads of hard disk drives. As the areal density of hard disks approaches 1 Tb/in2, the size of read head shrinks in all three dimensions: track width (TW), shield-to-shield spacing, and stripe height. In current fabrication processes for magnetic read heads, an ion-beam-etch (IBE) process is used to define the track width. In general, fabrication of a contiguous junction (CJ) involves etching out the CPP sensor (TMR or GMR) by the IBE process to designed TW and junction angle. Subsequently, a thin insulator and permanent magnetic film are deposited along the TW direction to stabilize the sensor. For a robust product performance, the TW dimension as well as the sidewall (junction) angle sigmas have to be tightly controlled.
In current fabrication processes, TW and junction angle sigmas are relatively large. One contributing factor for these variations is the beam steering/divergence associated with the IBE process. Another important contributing factor is the stability of the photoresist used for patterning. Another approach involves using a reactive ion etch (RIE) process used to get junction profiles without the in-board/outboard asymmetry normally associated with the IBE processes and their characteristic beam steering/divergence. The use of the RIE process can significantly reduce the junction angle sigma compared with the IBE process. Also, the TW sigma is expected to be reduced due to the use of tantalum (Ta) hard mask instead of a photoresist.
Recently, a new magnetic RIE process has been introduced that is specifically aimed at etching of magnetic multilayer structures. In this RIE process, an etch-stop layer 120 made of pure Ta with a thickness greater than 30 Å is put at the bottom of a sensor stack (comprising a bottom sensor layer 130, a barrier layer 140, free layers 150, 160, and a capping layer 170) in order to achieve a flat etching profile as shown in FIG. 1A.
However, the shield-to-shield spacing is reduced, the current AFM seed layer used in MgO TMR sensors is changed to CoFeB/NiFe magnetic seed layer, which is not effective as RIE etch-stop layer when using CH3OH etchant gas, for example. This gives rise to the formation of skirts 105 at the bottom of the sensor stack as shown in FIG. 1B. The formation of the skirts 105 is not desirable, as they hamper a good alignment between the free layer of the sensor and the hard bias used for stabilization.